Well, extra time (possibly the biggest barrier), money (but really not much) and people. With social media being so time intensive, and best results coming from directly being involved, the barriers to entry get higher as a business gets smaller, but not insurmountable for those willing to put in the time, especially since it can be done at all hours, and some of the most effective times to connect via social media are very early in the morning and very late at night.
In the future it is going to be critical for small businesses to decide where they want to put their scarce resources so they can maximize their ROI. With the ability to be building relationships, trust and loyalty at your fingertips, at scale, and for anyone to do no matter your budgets, the question really becomes… can you afford to not to.
And with Social the most important thing to remember is… IT TAKES TIME! So start now and figure it out as you go… or you won’t have the reach and influence you want, when you need it. Later will always be well… LATER.